window.onload = function() {
  var uAgent  = navigator.userAgent.toUpperCase();
  if (uAgent.indexOf("NT 5.1") >= 0) {
    if (uAgent.indexOf("MSIE") >= 0) {
      document.getElementsByTagName("body")[0].style.padding = "0 0 0 1px";
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    if (uAgent.indexOf("FIREFOX") >= 0) {
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  var dateObj = new Date();
  var s = dateObj.getSeconds();
  var img_number = s % 3;
  document.getElementById("cover_img").style.background = "url(/img/cover_img" + img_number + ".jpg) no-repeat top center";
}